Be explicit that Android examples need a Git checkout (#2465)

Without this, students will see subtle differences between what is on
the slides and what we have in the code (see #2464 for an example).

src/android/setup.md

@@ -13,6 +13,10 @@ Please see the
 [Android Developer Codelab](https://source.android.com/docs/setup/start) for
 details.
 
+The code on the following pages can be found in the
+[`src/android/` directory](https://github.com/google/comprehensive-rust/tree/main/src/android)
+of the course material. Please `git clone` the repository to follow along.
+
 <details>
 
 Key points: